@node Running Pre-Scheme as Scheme
@section Running Pre-Scheme as Scheme

@stindex prescheme
@stindex ps-memory
@stindex ps-receive
@stindex ps-record-types
@stindex ps-flonums
To facilitate the operation of Pre-Scheme systems within a high-level
Scheme development environment, Scheme48 simply defines the
@code{prescheme}, @code{ps-memory}, @code{ps-record-types},
@code{ps-flonums}, and @code{ps-receive} structures in terms of Scheme;
Pre-Scheme structures can be loaded as regular Scheme structures
because of this.  Those structures and the interfaces they implement
are defined in the files @file{scheme/prescheme/interface.scm} and
@file{scheme/prescheme/package-defs.scm} from the main Scheme48
distribution; simply load these files into the @embedref{Module
commands, config package} before loading any Pre-Scheme configuration
files.

The Pre-Scheme emulation layer in Scheme has some shortcomings:

@itemize
@item No more than sixteen megabytes can be allocated at once.

@item More than thirty-two or sixty-four or so allocations result in
addresses overflowing bignums, which deallocations does not affect.

@item Flonum memory access is unimplemented.  (Flonum arithmetic works,
though.)

@item The layer is very slow.
@end itemize
